This invention relates to a power source for an electric discharge machining, and more particularly to an electric discharge machining power source for performing a semi-mirror-finish machining operation with a surface roughness of 1 .mu.m Rmax or less.
In a high frequency electric discharge machining method, the average machining voltage is zero (0), and therefore there is no occurrence of chipping due to electrolytic action. Furthermore, the polarity is altered every half cycle of electric discharge thereby resulting in the change of the discharge point every electric discharge. Hence, the surface machined by the method is excellent in quality; that is, the high frequency electric discharge machining method is excellent in machining performance.
FIG. 1 is a circuit diagram showing a conventional power source for an electric discharge machining apparatus disclosed in Published Unexamined Japanese Patent Application (OPI) No. 260915/1986. In FIG. 1, reference numeral 1 designates a DC source; 2, a current limiting resistor; 6, a gap between an electrode and a workpiece facing each other; 3, a stray capacitance existing in a current supplying line and the gap 6; and 4, a stray inductance existing in the current supply line and the gap 6; 7, a switching element; 8, a drive circuit for driving the switching element 7; and 9 and 10, a coupling capacitor and a coupling coil, respectively, the coupling capacitor 9 and the coupling coil 10 forming a series circuit which is connected between the switching element 7 and the gap 6.
FIGS. 2 and 3 are equivalent circuits of the electric discharge machining power source shown in FIG. 1 for a description of the operation of the circuit.
Now, the operation of the electric discharge machining power source thus constructed will be described. In the case where the inductance of the coupling coil 10 is sufficiently larger than the stray inductance 4, when the switching element 7 is turned off, the circuit is regarded as a series circuit consisting of R.sub.1, C.sub.1, L.sub.1, C.sub.2 and the DC source as shown in FIG. 2, as a result of which C.sub.1 and C.sub.2 are charged by a current as indicated by an arrow in FIG. 2. When, under this condition, the switching element 7 is turned on, the circuit is changed into a series circuit consisting of C.sub.2, L.sub.1 and C.sub.1 as shown in FIG. 3, so that C.sub.1 and C.sub.2 are discharged as indicated by an arrow in FIG. 3. The switching element 7 is turned on and off by the drive circuit 8 at several mega-hertz (MHz), so that a high frequency voltage is developed across the gap 6, to perform an electric discharge machining operation.
In general, ##EQU1## where L is the stray inductance, C is a stray capacitance, E.sub.p is a current peak, T.sub.p is a current pulse width, E.sub.o is a gap voltage, and E.sub.a is an arc voltage. Therefore, it is well known in the art that the discharge energy is decreased as L and C decrease.
The stray capacitance 3 is the sum of the above-described capacitance existing in the current supplying line and the capacitance of the gap 6 (between the electrode and the workpiece). A machined surface excellent in quality, 1 .mu.m Rmax or less in surface roughness, can be obtained by setting the stray capacitance 3 to 1000 pF or less.
The conventional electric discharge machining power source is constructed as described above. Therefore, it is indispensable to decrease the stray capacitance 3 in order to obtain a machined surface excellent in quality. However, in practice it is considerably difficult to reduce the stray capacitance to less than 500 pF. And accordingly, it is impossible to obtain a machined surface which is 0.5 .mu.m Rmax or less in surface roughness.
When, in an electric discharge machining operation, the machining gap and the machining area are changed, or when the conditions of electric discharge are changed, then the impedance of the interelectrode gap 6 is greatly changed. This results in great change in the output. Consequently, the conventional device is disadvantageous in that the electric discharge machining operation is considerably unstable, and is low in reproducibility.